Thank you for your clarifications. To paraphrase, you say that:
a. A Full Node is a Full Node even when incoming traffic is ZERO,
b. Even when "a" is the case, one is supporting the network's consensus mechanism.
I never say point "a". Incoming connection (another node connect to your node) and incoming traffic (receiving data) are 2 different thing.
1. Why bother enabling 8333 port forwarding? There must be a reason why bitcoin.org recommends
"The bandwidth sharing guide describes installing Bitcoin Core in detail as well as opening port 8333 to allow other Bitcoin programs to download blocks and transactions."
Adding what @ranochigo said. Estimated total full node is about 70K[1], while estimated total full node which accept incoming is only about 19.5K[2].
[1]
http://luke.dashjr.org/programs/bitcoin/files/charts/software.html[2]
https://bitnodes.io/dashboard/90d/